Those of you who remember the article about togas that ran last year in these pages (or see http://www.mmdtkw.org/VToga.html) will know that the toga was originally an Etruscan invention and was nothing but a huge, five meter by two meter, woven cloth of raw wool that was wrapped artistically around the human frame for warmth and theatrical effect. Some ancient historians guess that, before the advent of good metal needles in these parts, all they could do was wrap up -- patent nonsense, of course: sewn garments first appeared millennia earlier. But, for whatever the reason, togas were routinely worn (sans undergarments) by Etruscan men and women and by their male and female Roman cousins in the earliest days of Rome. Early on, togas went out of style for the perpetually fashion-conscious Roman women, except for the "working girls" of the streets, who wore them as a kind of uniform. Roman men continued to wear them until the end of the Empire, although by the end they had shrunk down to the size of a ceremonial cape.
Roman men, after the first few years of toga wearing, figured out that finer cloths chafed less than even the lightest raw wool togas, so almost everyone (excepting the Catos and their ilk, who were always trying to make a point) took to wearing tunics under their togas. A tunic was just a woven sleeveless or short sleeved pullover that ended just at the knee. It was usually belted or tied at the waist, and you could pull it up around the belt to cool off vital parts on hot summer days. In latter days long-sleeved tunics also became acceptable -- after Roman men got over the idea that long sleeves were effeminate. Finally, some men (and women) wore a kind of loin-cloth, called a subligar or subligaculum, which, as its Latin etymology suggests, was "tied on under", i.e., knotted on both sides. The whole ensemble might be called a "synthesis", equivalent to the English word "outfit", although "synthesis" might also be used to specify a simpler dinner costume donned to avoid spilling wine on your toga or simply because it was more comfortable.
Plebians wore sandals for most ceremonial occasion, patricians wore red boots so everyone would know who they were, and senators wore similar boots in black. Weather was an important footwear consideration, and fur might be stuffed in for warmth, or socks of sewn woven cloth might be worn. (The English word "socks", comes directly from Latin where a "soccus" was a loose-fitting ankle-length soft leather slipper.) Soldiers wore heavy boots often with hobnails on the bottom for traction and strength -- but their whole "synthesis" was specialized, and easily recognized from countless modern movie images -- or you can see military garb on men and the occasional woman posing with tourists near the Colosseum of Trajan's column here in Rome.
Slaves and foreigners, of course, weren't allowed to wear togas. The former wore tunics, or, for hot summer work, the simple loincloth. Foreigners might wear tunics or their own "national dress". Manual laborers and freedmen wore pretty much the same clothing as slaves, although in later Empire times, freedmen could attain citizenship and with it the toga.
Women, as mentioned above, quit their togas and, when they did, they adopted other styles, which were essentially Greek. They, too, wore tunics, and they were of two basic and long-lasting styles. The peplos was simply two pieces of cloth that reached from a woman's shoulders to her ankles. The edges were sewn together from the armpit to the knee (or to the hip, for the more daring upper-class types). Two broaches fastened the top edges together, and a belt or cord was tied around the waist. The chiton, also a Greek style, was more common and also was made of two sewn together panels, but they could be much wider. Six, eight, or even more small pins were arrayed outward from the neckline along the upper edges extending down the arms for a sleeve-like effect, and cords or ribbons were wound below the breasts and around the waist. At the time of marriage, a woman was entitled to don a stola, a long sleeveless wool tunic, over her regular peplos or chiton. This was, in fact, a ceremonial garment equivalent to the male toga. A respectable matron would top this ensemble with a cloak called a palla, which was draped almost the same way that men wore their togas. All these layers were probably cozy in winter, but were just as probably quickly cast off in summer after ceremonial proprieties were finished. For most of the duration of the Republic and Empire, respectable female garb was almost uniform -- unvarying over time and almost always a few drab colors. Makeup, jewelry, and elaborate hairstyles were all that respectable women had in order to vary their appearance. Of course, there were a lot of not-so-respectable women for the men to leer at -- courtesans, lower class prostitutes, and rebellious upper class wives and daughters. Female members of imperial families were often the least scrupulous in their regard for rules of feminine attire.
As the Empire expanded eastward, finer cloths and sheer silks were added to the feminine wardrobe, and, at least in paintings and statuary, some striking effects were achieved. It's hard to tell how often such garments were worn in public, but in some instances comments were recorded -- always on the prudish side, of course, because those who appreciated such spectacles were too busy chasing after the ladies in question, and didn't want to stop to take notes. Feminine undergarments included the tied subligar below and a cloth or leather strophium or mamillare for breast support. (A fine well-preserved leather "bikini bottom" of the Roman period has been recovered in Britain, but it may have been a "barbarian" artifact.)
Clothes made both the man and the woman. Class distinctions were strictly enforced -- insignia stripes, colors, styles, and the amount you could spend on clothes were all regulated pretty consistently by class-based sumptuary laws, and there were always plenty of jealous informers to report if anyone strayed from the rules. There were occasional prosecutions, but the most important punishment for "dressing up", that is, above your station, was always the possibility of withdrawal of "patronage". Everyone was dependent, in one way or another, on persons of higher class, and if you offended them by wearing clothes that challenged their superiority, you could be quickly cut off. It was a totally non-democratic system that kept everyone in his or her own place.
That "his or her" was also important. Cross-dressing was not unknown, but you could get in serious trouble for it -- unless, of course, you were in the Palace. Most non-palace instances were resolved within the family, and, if that didn't stick, by disgrace and withdrawal of patronage. Even in the Palace, you were not completely immune to disapprobation. Young emperor Heliogabalus, born in Syria and a sun-worshipper and cross-dresser, reigned only four years before being dispatched (222 AD), but it may really have been because he submitted to a slave rather than because of the feminine clothes he wore while doing it. An earlier famous case was that of the demagogue Publius Clodius Pulcher who was prosecuted for infiltrating the annual women-only "Bona Dea" rites in 62 BC by dressing as a woman. The charge, however, was sacrilege -- for being there, not for the clothes he wore to get in. Even though the prosecuting attorney was the famous Cicero, Clodius got off -- by massively bribing important Senators. But Pompeia, the hostess of the affair and the wife of Julius Caesar, was divorced and disgraced, because, contrary to Caesar's famous dictum, she was not "above suspicion" -- she was rumored to have colluded with Clodius to get him into the ceremony as well as into her bed. Caesar, of course, was, meanwhile, enjoying the unparalleled benefits of the double standard of the day and notoriously chasing anything in a chiton. But enough of this gossip!
If you want to wear Roman clothes today, they are fairly easy to make, and there are lots of instructions on the Internet -- much of it aimed at Roman period re-enactors. They are heavily into military garb, because, like most reenactors, they really like to do the battles better than anything else. Most of their sites do, however, have sections on "civilian clothes." There are some links below and also some to commercial suppliers. The re-enactors are scrupulously accurate and authentic, but the commercial shops tend to stray into the "show-biz" side, providing lots of props and costumes for stage, screen, and TV productions. There are also links to scholarly sites that describe and often present pictures and drawings of what the Romans really wore.
One further note: "Rome" lasted for about a thousand years (Romulus until the rapid decline that started with the departure of Constantine) and several hundred years more if you count the post-Constantine "Western" and "Eastern" emperors -- even more if you count those "Holy Roman" pretenders. "Rome" also, at times stretched from the North Sea, across the Mediterranean, and down the shores of the Red Sea and from the Atlantic well into southwest Asia. During all those years and across that broad swath of Europe, Asia, and Africa, "Romans" wore a lot of "non-standard" clothes. It's safe to say, however, that, in all that time and territory, it was always clear what were "Roman clothes" and what was foreign.
Scholarly info from Bill Thayer's "Lacus Curtius" site: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/secondary/SMIGRA/Daily_Life/Clothing/home.html
Professorial (Barbara McManus) pages, with pix and diagrams on men's clothing: http://www.vroma.org/~bmcmanus/clothing.html and women's: http://www.vroma.org/~bmcmanus/clothing2.html
A Glossary of Roman clothing from the respected Diotima site (U. of Kentucky): http://www.uky.edu/ArtsSciences/Classics/jlsgloss.html
The Legio XX (Twentieth Legion -- based in Washington DC) reenactors' site -- one of many on the Internet: http://www.larp.com/legioxx/ with lots of links to the world of Roman re-enactment and to their own "civilian clothes" page at http://www.larp.com/legioxx/civcloth.html and a page full of links to commercial suppliers of all kinds of Roman clothing and gear.
Ancient Roman costume links from "The Costumer's Manifesto" -- more than anyone will ever need at http://www.costumes.org/pages/romanlnx.htm -- is only a small part of one of the net's largest costume sites: http://www.costumes.org/
Go to http://www.mmdtkw.org/Veneto2001.html for other articles.